This invention relates to the removal of particulate matter from ammonium polyphosphate solutions which are prepared from impure wet-process phosphoric acid. More particularly, it relates to a new and improved process for the flocculation, flotation, and removal of suspended, black, carbonaceous impurities from aqueous solutions of ammonium polyphosphate which are produced from wet-process phosphoric acid.
Wet-process phosphoric acid is produced by the reaction of phosphate rock, particularly calcium phosphate, with sulfuric acid and separating the resulting crude phosphoric acid from the precipitated calcium sulfate and other insoluble materials. This crude acid ordinarily contains about 30 to 32% P.sub.2 O.sub.5 when produced (filter-grade acid) and is normally concentrated to a P.sub.2 O.sub.5 concentration of about 52 to 54% (merchant-grade acid) before shipment. Concentration of the wet-process acid prior to shipment not only serves to increase the P.sub.2 O.sub.5 content, but also has the effect of increasing the concentration of impurities that are present.
Crude wet-process phosphoric acid contains varying amounts of impurities such as iron, aluminum, calcium, magnesium, and other metals in the form of their phosphates, sulfates, or fluorides, depending on the characteristics of the specific phosphate rock which is used as the starting material. In addition to these inorganic impurities, organic materials are also commonly present as contaminants and are derived from organic substances originally present in the phosphate rock and from organic chemicals utilized in benefication of the rock prior to treatment with sulfuric acid. A portion of these impurities are in solid form and result from imperfect filtration, from precipitation during concentration, or from post-precipitation. Other impurities are soluble and remain in solution.
Phosphate rock which is utilized for the preparation of wet-process phosphoric acid is often heavily contaminated with both soluble and insoluble organic material. Although organic material can be removed by calcination of the phosphate rock prior to use, such processing is becoming increasingly unfeasible as a result of the large energy requirement involved. This is illustrated by the fact that such calcination requires an expenditure of about 3 million Btu per ton of rock. Consequently, furnace-grade phosphoric acid, which is prepared from calcined phosphate rock, is becoming increasingly expensive and impractical for use in applications such as the preparation of fertilizers.
In the preparation of liquid fertilizers, it has been found desirable to concentrate merchant-grade phosphoric acid to higher P.sub.2 O.sub.5 values. By subjecting the merchant-grade acid to evaporating means, either at atmospheric or reduced pressure, the P.sub.2 O.sub.5 content can be increased from about 52 to 54% to a value in the range from about 60 to 76%. This concentration results in a molecular dehydration of the phosphoric acid and conversion from the orthophosphate form to the polyphosphate form. Occurring concurrently with the molecular dehydration of the merchant-grade acid is the charring of dissolved organic impurities at the elevated temperatures required. The resulting product is a black phosphoric acid solution which contains a substantial amount of polyphosphate and also a plethora of suspended, black carbonaceous particles which originate from the organic impurities in the merchant-grade starting material. Although it would be highly advantageous to clarify this black phosphoric acid before its combination with other components, as in the manufacture of fertilizer solutions, no practicable process for doing this has yet been devised.
Wet-process phosphoric acid is widely used in the manufacture of ammonium polyphosphate fertilizer solutions. Two basic methods are utilized for this purpose. One method, as set forth in U.S. Pat. No. 3,044,851, involves the dehydration of merchant-grade wet-process phosphoric acid followed by reaction with ammonia in a subsequent step. Alternatively, as set forth in U.S. Pat. No. 3,382,059, filter-grade or merchant-grade wet-process phosphoric acid can be reacted with anhydrous ammonia, and the autogenous heat of reaction utilized to dehydrate and convert the orthophosphate starting material to ammonium polyphosphate in a single step. Unfortunately, both of these methods produce ammonium polyphosphate solutions which are highly discolored due to the presence of black carbonaceous particulate matter. This particulate carbonaceous material is derived from organic impurities in the wet-process phosphoric acid which char and are rendered insoluble during the dehydration which converts the orthophosphate starting material to a polyphosphate product. Although furnace-grade phosphoric acid can be utilized, in place of the wet-process material, to produce clear ammonium polyphosphate solutions which are free of black, particulate carbonaceous material, a substantial economic penalty accompanies the use of furnace-grade acid.
Liquid fertilizers having compositions similar to those of standard dry mixed fertilizers are well known in the industry and are increasing in popularity. Such solutions are superior to dry mixed fertilizers in that the cost of evaporating water and bagging the product are eliminated and soil application is greatly simplified. In addition, the use of liquid fertilizers eliminates difficulties due to segregation and caking which are often encountered in the storage of dry fertilizers. Although ammonium polyphosphate solutions prepared from wet-process phosphoric acid, as detailed supra, are currently the preferred source of liquid fertilizers containing nitrogen and phosphorous, they suffer from the serious deficiency of an inky black color which is due to suspended, black carbonaceous particulate matter. Filtration of the black ammonium polyphosphate solution is not practicable since the carbonaceous particles coalesce on the filter to generate a sticky impermeable layer which prevents further filtration. In addition, this suspended carbonaceous material will not spontaneously settle under conditions which would permit its removal on a large scale.
Black ammonium polyphosphate fertilizer solutions, which are prepared from wet-process phosphoric acid, are objectionable since the dark coloration masks the presence of precipitated solids which can result in plugged equipment and difficulties in the application of such material to the soil. In addition, this type of black fertilizer solution is less aesthetically pleasing than a clear ammonium polyphosphate solution which does not contain suspended black carbonaceous material, but which is otherwise the same. In view of these factors, consumers are becoming increasingly resistant to the use of black ammonium polyphosphate fertilizer solutions or other black fertilizer solutions derived from them. At the present time, the consumer will not use these black liquids except in the case of extreme emergency. Consequently, a large price differential exists between the black ammonium polyphosphate solutions and clear green solutions of the same analysis.
It has previously been demonstrated that suspended, black carbonaceous material can be removed from aqueous ammonium polyphosphate solutions by a flocculation and flotation process. U.S. Pat. No. 3,630,711, is directed to a method for clarifying black 10-34-0 grade ammonium polyphosphate solutions. This method involves the addition of from about 0.1 to about 0.6% by weight of a water insoluble aliphatic amine to the black solution followed by vigorous agitation of the mixture after which the carbonaceous material is permitted to flocculate and float to surface as a froth. The resulting froth is then conveniently removed.
An improved flocculation and flotation process for the clarification of black, aqueous ammonium polyphosphate solutions is set forth in U.S. Pat. No. 3,969,483. This improved process involves the combination of two flocculating agents, one of which is either an aliphatic amine or an aliphatic amine acetate and the other is a quaternary ammonium chloride. These two flocculating agents act together in a synergistic manner and produce flocculation at addition rates which are significantly lower than those required by the process disclosed in U.S. Pat. No. 3,630,711. Even with this improved process, however, it is still necessary to use a total of about 0.1% of these two flocculating agents in order to produce a clear ammonium polyphosphate solution having a light absorbance below 0.3 (relative to distilled water at 560 nm). This process also requires vigorous agitation of the mixture of black ammonium polyphosphate solution and flocculating agents. In addition, this process is carried out at elevated temperatures in the range from about 165.degree. to about 190.degree. F. which results in the substantial and undesirable loss of polyphosphate content in the solution.